NEW YORK — This Tony season, an unprecedented number of movie and Hollywood stars under 40 are hitting Broadway for the first time.

"Scrubs'" Zach Braff just made his debut in Susan Stroman's staging of Woody Allen's comedy-musical "Bullets Over Broadway." Golden Globe winner and Oscar nominee Michelle Williams is trying out her singing talents — and a British accent — as Sally Bowles in the Roundabout's sort-of revival of "Cabaret."

But nothing epitomizes this influx more than the trio of Broadway rookies bringing a burst of star power — and their fan followings on social media — to Tony-winner Anna D. Shapiro's staging of the John Steinbeck classic "Of Mice and Men" about Depression-era migrant workers in California.

The show, which hasn't been seen on Broadway in four decades, opens Wednesday at the Longacre Theatre with James Franco (1.9 million Twitter followers) as the hustling George Milton, Leighton Meester (1.4 million followers) in the ingénue role of the wife of the boss' son Curly, and a padded-out Chris O'Dowd (with a comparatively measly half a million followers) as the slow-witted giant Lennie Small.

"I'm a Luddite so I don't really know what Twitter or Instagram is, except that young people do it," said Shapiro, who actually has experience with youth-skewing shows, having directed "The Mother... With the Hat" starring Chris Rock three years ago, as well as the more traditional "August: Osage County."

"But I know," the director added, "I've never seen the theater looking that young."

While Broadway productions increasingly cast a young Hollywood star in a major role, they rarely have them as the three leading characters.

Showcasing a group of young performers like Shapiro's "Of Mice and Men" stars — the oldest of the three is Franco at 35 — can mean a potential windfall of enthusiasm, both from the actors and theatergoers.

All three actors (they're joined in the ensemble by Broadway veteran Jim Norton as Candy and newcomer and "True Blood" star Jim Arrack as Slim) have been particularly devoted to promoting the show to fans. Franco updates his Twitter and Instagram accounts as many as several times a day with show-related posts. "MICE AND MEN FEVER," he wrote in one recent tweet, alongside a photo taken of fans outside the theater. Selfies, indeed, are the new bus-stop ad.

But there are also pitfalls to Hollywood casting. The production has to take the stars' outsized reputations into account as they deal with everything from first-timer hiccups to scheduling snafus. Last year, Shia LaBeouf, known for the blockbuster "Transformers" movies, made an abrupt departure before the previews of "Orphans" over what the producers called "creative differences."

For the "Of Mice and Men" production, Shapiro said she was very conscious of the fact that her actors would be well-known to the audience as someone else — Meester as the slippery Blair Waldorf on the CW's long-running "Gossip Girl," O'Dowd as the Irish-born actor who's been blowing up in the U.S. over the last few years in "Bridesmaids," "Girls" and his own HBO show "Family Tree." And then there's the public-private persona known as James Franco.

Shapiro cast O'Dowd, she said, because he was a nuanced actor physically proportioned like Lennie, and she admired Meester's subtlety within her small-town beauty. (Franco was already onboard.)

Still, the issue has given her pause. She nixed a shirt-removal scene for George because she thought James Franco disrobing on a theater stage every night might make the Internet explode. She quickened the pace of George and Lennie's entrance to cut down on the distraction factor as well.

Franco said he believes that in preview performances so far audiences have noted the star power on stage, but only briefly. "It's really the initial moment," he said. "We give them a few seconds, and then it's, 'Now we're going to play the parts and get on with the show.'"

Meester said she's tried to focus on her own efforts and not be sidelined by the issue. "It's not about [the audience] so much as how I feel," she said. "It doesn't matter if people know something else I've done; I'm just happy to be doing something new."

But O'Dowd said he doesn't mind if audiences think of other roles, such as his nice-guy cop wooing Kirsten Wig in "Bridesmaids."

"I'm very conscious of what people know of me and I don't in any way try to get rid of that," he said. "I feel I can use people's expectations of me a little. With Lennie's character we can be as funny as we want and it only adds to it. It only makes it sadder at the end," he said, alluding to the play's dramatic turn.

Offstage, distractions can also take their toll, as it seemed to when Franco's flirtatious communication via Instagram with a female fan who had not yet turned 18 surfaced a couple of weeks ago.

And the schedule, though more regular than the erratic timetables of film shoots, can be tricky.

Franco continues flying to Los Angeles on his day off to teach a class. (Braff soon will similarly spend nearly every day off during the next few months flying to various cities for screenings promised to some of the 47,000 people who donated to his new movie, the L.A.-set dramedy "Wish I Was Here," on Kickstarter.)

When Shapiro and Franco first began working together, they had what she describes as "come-to-Jesus moment" in which she laid out that his day off was his business but he needed to be fully present when he was at the theater; he nodded eagerly and she said it's never been an issue since.

"James sometimes does things people read as dilettantish," she said. "But there isn't a casual bone in his body."

Working with Broadway newbies, she noted generally, can be a double-edged sword. "There are things you get bogged down in that don't normally get bogged down, but they also trust you in a different way, because what choice do they have?" she said with a laugh.

Franco said the absence of a camera meant adjusting his skills to work from every angle and to make everything count throughout the staging as opposed to just capturing a single moment that can be inserted into a final cut.

"Everything kind of depends on another actor," he said as he sat in the theater with O'Dowd and Meester. "You can't just do your performance and be fine. You want everyone to be awesome not just for the piece but for yourself."

O'Dowd offered a mischievous look when asked if he thought skeptics might view the casting of him, Franco and Meester as a gimmick.

"I haven't noticed the backlash yet," the actor said with a wry smile as he sat in a Broadway theater a few hours before he would take the stage. "I'll keep my eyes open."

Certainly there are marketing benefits to bringing on younger celebs, but for the actors, the chance to take on an unexpected challenge is what motivates them. Broadway gives screen actors the opportunity to showcase their skills in real time with a classic text and without the distortions of the Hollywood editing process.

For "Of Mice and Men," the actors believe they can be part of something culturally important: introducing a difficult drama to a new audience.

"I'm very conscious that this will be this generation's 'Of Mice and Men,'" O'Dowd said.

"And because we're selling well, the way commercial theater works it will be on again in 10 years," he added. "With the kids from 'Modern Family.'"